It is standard practice in the manufacture of many semiconductor devices, and other devices, to provide a thin passive layer or coating of a chemically nonreactive or low reactivity material to protect the semiconductor junctions and layers from degradation by contact with oxygen, fumes in the air, moisture, etc., and from damage by contact during packaging and handling.
The production of silicon oxide and silicon nitride coatings application to semiconductor devices and to other devices is well known, and there is considerable literature on the subject. The general principles underlying the formation of thin films are described in HANDBOOK OF THIN FILM TECHNOLOGY, Maissell, Leon I. and Glang, Reinhard, editors, McGraw Hill Book Company, New York, 1970, and the general technology for processing silicon-based semiconductor devices in described in SILICON PROCESSING FOR THE VLSI ERA, Wolf, Stanley, and Talbert, Richard N., editors, Lattice Press, Sunset Beach, Calif., 1986, which includes a discussion of thin film technology.
Silicon dioxide is commonly used as a film-forming material to protect semiconductor devices. Thermally grown films of silicon nitride, Si.sub.3 N.sub.4, however, have a number of advantages over silicon dioxide. For example, silicon nitride films tend to have self-limiting growth kinetics, and therefore their thickness is easily controllable. In addition, silicon nitrides are effective barriers to moisture, oxygen and sodium and other gas and chemical diffusion. MOS devices fabricated with silicon nitride films show large values of gain and reduced hot electron effects.
Silicon nitride films are amorphous insulating materials and serve as final passivation and mechanical protective layers for integrated circuits, as a mask for selective oxidation of silicon, and as a gate for MNOS devices. Silicon nitride is known to have a high dielectric constant and a low compressive stress and can be deposited with low pinhole densities.
Thermal silicon nitride films are generally grown by the high temperature nitridation of silicon in pure ammonia or an ammonia plasma. Silicon nitride films can also be prepared by plasma anodic nitridation using a nitrogen-hydrogen plasma. The thermal nitridation of silicon dioxide films results in the formation of a nitrided-oxide or a nitro-oxide film, the composition of which is found to vary considerably with the depth of the film.
Two techniques are most generally used to deposit silicon nitride, namely the low pressure chemical vapor deposition (LPCVD) process and the plasma enhanced chemical vapor deposition (PECVD) process. These processes are described in SILICON PROCESSING, supra, Chapter 6. Very briefly, the LPCVD process is typically carried out at from about 0.25-2.0 Torr and at temperatures of 700.degree. C. to about 825.degree. C. The PECVD METHOD relies substantially solely on thermal energy to initiate and sustain chemical reaction, using a radio frequency (rf) induced glow discharge to transfer energy into the reactant gases, allowing the substrate to remain at a lower temperature than is the case in the LPCVD, and the less used atmospheric pressure chemical vapor (APCVD) processes. The plasma (which is more correctly described as a glow discharge) is generated by the rf field creating free electrons which collide with the gas molecules, causing gas-phase dissociation and ionization of the reactant gasses. Upon being adsorbed on the substrate, the film precursor is bombarded by electrons and ions and are further dissociated and rearranged until, ultimately, a film consisting essentially of silicon nitride results.
The production of silicon nitride films by the reaction of silane, SiH.sub.4, ammonia and hydrogen is quite well known. The analogous reaction of alkylsilanes is also known. For example, Great Britain Patent No. 1,146,383 describes the production of silicon nitride films by the reaction of trimethylsilane in ammonia, East German Patent No. DD 90,185, describes the production of a silicon nitride passivation layer by the vapor phase reaction of trimethylsilane, and Fischer, Z. Phys. Chem. (Leipzig), V. 255, N. 4, pp. 773-86 (1974) describes the hydrolysis of methylsilane and the reaction with ammonia. There are a number of patents and publications which describe variations of this technique involving the vapor phase reaction of tetramethylsilane with ammonia, or comparable silane-amine compounds.
Nelson, U.S. Pat. No. 4,158,717 reviews the state of the art of producing silicon nitride films and describes the production silicon nitride films as protective and anti-reflective coatings and for masking semiconductive devices by a plasma discharge in azidotrimethylsilane, (CH.sub.3).sub.3 SiN.sub.3, pointing out that azidotrimethylsilane is easier to handle than silane.
Matsuda et al, U.S. Pat. No. 4,569,855, discloses the use of azidosilanes in a very low temperature (50.degree.-150.degree. C.) high energy light activated gas deposition process for forming silicon and doped silicon films. While reference is made to alkyl, aryl and alkoxy azidosilanes, there is no specific disclosure of the use of ethyl, propyl, or butyl azidosilanes, or to higher-carbon alkyl, aryl or alkoxy azidosilanes in the Matsuda et al patent. Since the greater part of the Matsuda et al process activation energy for decomposing the gas to form a film comes from very high energy light photons, e.g. U.V., X-ray or gamma photons, it would not be possible to predict from Matsuda et al what may result from the use of higher-carbon azidosilanes in a thermal-activation method at higher temperatures. Matsuda further states that only some silicon deposition would result from a thermal decomposition process.
Silicon nitride-silicon oxide films are produced by the gaseous reaction of alkylsilane or arylsilane with ammonia or a volatile amine and a gaseous hydrocarbon such as methane, propane, etc. according to a process described by Bogh and Mirbach, U.S. Pat. No. 4,091,169.
There are also a number of processes described involving production of silicon nitride or silicon nitride-containing films, fibers or structures using a variety of silicon-containing compounds such as polycarbosilanes, siloxanes, etc., e.g. European Patent Application No. EP167230 A2, Jan. 8, 1986.
Plasma-deposited silicon nitride films from hexamethyldisilazane have been described by Janca, J. et al., Scr. Fac. Sci. Nat. Univ. Purkynianaebrun., V. 14, N. 1-2, pp. 27-33 (1984). Silicon nitrides have also been produced from polysilazanes, U.S. Pat. No. 4,612,383, Laine, Richard N., Sept. 16, 1986 and by the reaction of trichloromethylsilane in the presence of nitrates, Japanese Patent Application No. 78464733, Apr. 21, 1978, Indo, Hiroshi, et al.
Polymethylsiloxane nitridation has also been carried out to produce silicon nitride, Japanese Patent No. 7993699, July 24, 1979, Motoi, Soichiro.
Azidosilanes, the characteristics and preparation thereof, have been described; see, for example, Wiberg, Int. Symp. Organosilicon Chem., Sci. Commun., pp. 232-35, 1965; Liu, Sheng-Lieh, et al., J. Chin. Chem. Soc. (Taipei), V. 17, N. 4, pp. 229-34 (1970); Wiberg, Nils, et al., J. Organometal. Chem., V. 22, N. 2, pp. 349-56 (1970).
As mentioned, it is known to produce silicon nitride films from azidosilanes under severe reaction conditions such as occur in a plasma discharge such as described, for example, by Nelson, supra. A glow discharge is a self-sustaining type of plasma, i.e. a partially ionized gas containing an equal number of positive and negative ions, as well as some non-ionized gas particles. A glow discharge is created by the application of a voltage differential between the material with respect to which the glow discharge is applicable and another electrode. Nelson utilizes the well-known technique of using high frequencies, e.g. 13 MHz, for applying high voltages and enhancing plasma generation. As the voltage is increased, a point is reached in which gas breakdown occurs and there is current flow, vis-a-vis free electrons and gas ions, between the two electrodes. Each new ionization event takes place closer to the positively charged anode, as the electrons are accelerated toward and in collision with the anode. When a sufficient number of electrons are available to maintain the discharge, the discharge becomes self-sustaining. Substantially all of the energy is applied to the source, and the ions created at the source by the application of the energy simply impinge upon the surface of the material to be coated. This phenomena is extremely important where the substrate upon which the film is to be deposited cannot be heated to very high temperatures. Nelson, supra, points out that the substrate can be at room temperature, though higher temperatures result in superior films.
An inherent characteristic of the glow discharge method, where the source is other than a single element, is the creation of a large number of ionized and non-ionized fragments of the source. Thus, plasma discharge operations differ very significantly from the thermal process. For example, it is well known that the thermal TEOS deposition takes place at temperatures in excess 650.degree. C., even in the absence of oxygen. A plasma TEOS film deposition, on the other hand, occurs at temperatures below 350.degree. C. in the presence of oxygen but does not produce a pure silicon dioxide. The film also contains significant amounts of hydrogen and is also contaminated with organic polymer residues. These carbonaceous impurities are only eliminated in a plasma/ozone process at temperatures around 400.degree. C. minimum in the presence of excess oxygen.